Soliton pulse transmission over waveguide fiber lengths

ABSTRACT

The invention proposes to increase the signal/noise ratio in a long-haul transmission system by:  
     filtering noise ( 1 ) outside the range of wavelength of the signals transmitted,  
     shifting ( 2 ) the wavelength of the signals transmitted, and  
     filtering the signals transmitted ( 5 ) that have undergone wavelength shifting.  
     The wavelength of the signals can be shifted by widening the spectrum of the signals or by optical phase conjugation.

[0001] The present invention relates to fiber optic transmission systemsand more particularly to optical links and networks having a very highcapacity and covering very long distances. By optical links with a veryhigh capacity is meant transmission systems providing a bit rate greaterthan 10 Gbit/s. By transmission systems covering very long distances ismeant systems covering propagation distances of the order of 5000 km ormore.

[0002] At present obtaining very high capacities on long systems is notpossible, apart from N*2.5 Gbit/s wavelength division multiplex systems,even though papers have been published reporting laboratory results thatare as yet incompatible with the requirements of real systems.

[0003] At present, RZ (return to zero) pulse transmission and NRZ (noreturn to zero) pulse transmission are widely used in long-haul fiberoptic transmission systems. One problem for this type of system is thatthe signal/noise ratio increases as the number of repeaters in thesystem increases, due in particular to amplified spontaneous emission(ASE) noise.

[0004] In the case of soliton signal transmission systems, to reduce theamplified spontaneous emission noise, and thereby to increase thesignal/noise ratio, it has been proposed, for example in EP-A-0 576 208,to use sliding guiding filter systems. This solution is based on theparticular nature of solitons and their capacity for selfregeneration.In other words, soliton signals track the sliding of the filters,whereas the amplified spontaneous emission noise is filtered out.

[0005] This solution also applies to types of signals other than solitonsignals. However, because it relies on self-phase-modulation of thesignals, it is difficult to implement for wavelength division multiplextransmission systems, because of crossed phase modulation betweenchannels. The passage of the signals through the sliding filters impliesa high level of self-phase-modulation, which goes hand in hand with ahigh level of crossed phase modulation. To obtain results with this typeof solution in a wavelength division multiplex transmission system itwould be necessary to separate the various channels well beyond the bandof a few nanometers available for the signals.

[0006] The transmission of signals in optical systems is also limited bynon-linear effects, such as the Kerr effect, the Brillouin effect, theRaman effect and four-wave mixing. G. P. Agrawal, “Nonlinear FiberOptics”, Academic Press, 1980 describes these non-linear effects. Theydepend on the level of noise in the optical fibers of the transmissionsystem.

[0007] The invention proposes a solution to the problem of increasingnoise, and in particular of amplified spontaneous emission noise in afiber optic transmission system. It significantly improves the qualityfactor of transmission systems, especially systems of very high capacityand covering long distances. The invention eliminates most of the noiseat the wavelengths of the signals transmitted and makes possible“linear” transmission that is not limited by the noise level; it alsomakes possible nonlinear transmission limited by the effects of thenoise. Some embodiments of the invention reduce the timing jitter of thesignals.

[0008] To be more precise, the invention proposes a noise-reducingdevice for a fiber optic transmission system, said device including

[0009] first means for filtering noise outside the range of wavelengthsof the signals transmitted,

[0010] means for shifting the wavelength of the signals transmitted, and

[0011] second means for filtering the transmitted signals that haveundergone wavelength shifting.

[0012] The device advantageously includes second wavelength shiftingmeans for returning the signals that have undergone the secondfiltration to their initial wavelength.

[0013] In one embodiment the wavelength shifting means include means forwidening the spectrum of the signals.

[0014] The wavelength shifting means preferably include optical phaseconjugation means.

[0015] In one embodiment the filtration means include a Bragg filter.

[0016] The invention also provides a fiber optic transmission systemincluding at least one such device.

[0017] The invention further provides a method of reducing noise in afiber optic transmission system, said method comprising the steps of

[0018] filtering noise outside the range of wavelength of the signalstransmitted,

[0019] shifting the wavelength of the signals transmitted, and

[0020] filtering the signals transmitted that have undergone wavelengthshifting.

[0021] In one embodiment the method further includes a second wavelengthshifting step for returning the signals that have undergone the secondfiltration step to their initial wavelength.

[0022] The wavelength shifting step advantageously includes widening thespectrum of the signal.

[0023] The wavelength shifting step can also include conjugation of thephase of the signal.

[0024] Other features and advantages of the invention will becomeapparent on reading the following description of embodiments of theinvention, given by way of example and with reference to theaccompanying drawings, in which:

[0025]FIG. 1 shows the spectrum of signals in a transmission system;

[0026]FIG. 2 shows the spectrum of the signals after a first step offiltration in accordance with the invention;

[0027]FIG. 3 shows the spectrum of the signals after a step ofwavelength shifting in accordance with the invention;

[0028]FIG. 4 shows the spectrum of the signals after a second step offiltration in accordance with the invention;

[0029]FIG. 5 is a diagram of a noise-reducing device in accordance withthe invention;

[0030]FIG. 6 shows the spectra of the signals for wavelength shifting byoptical phase conjugation;

[0031]FIG. 7 is a graph of the power of signals received in aconventional transmission system; and

[0032]FIG. 8 is a graph of the power of signals received in atransmission system using the invention.

[0033] The invention proposes to filter the signals transmitted and thento subject them to “non-linear hopping” so as to recover them in therange of wavelengths previously filtered, in which the noise has beenreduced. It proposes various ways to effect the non-linear hopping, inother words to obtain a wavelength shift for the signals transmitted.

[0034] FIGS. 1 to 4 show the spectra of the signals in a firstembodiment of the invention. In this embodiment, simple widening of thesignals transmitted is used to subject the signals to non-linearhopping. FIG. 1 shows the spectrum of the signals. The noise level is ofthe order of N₀ and the signal level is of the order of E. The signalshave wavelengths around λ_(c), which is typically 1550 nm, the range ofwavelengths in the transmission system extending from 1530 to 1580 nm.

[0035] The invention proposes firstly to filter noise outside the rangeof wavelengths of the signals. In the FIG. 2 example, the noise isfiltered around the wavelength λ₁, which is higher than the wavelengthλ_(c). A band-pass filter or a high-pass filter can be used to filterthe noise, for example. For RZ pulses centered at 1550 nm, whichtypically have a spectral width of 0.2 nm, the noise can be filteredaround a wavelength λ₁ that is about 1 nm above the center wavelength ofthe pulses. The width of the filter can be of the order of 0.3 nm; it isadvantageously chosen so that it does not reduce the power of thesignals transmitted by more than 0.5 dB.

[0036] The invention then proposes to subject the signals transmitted tonon-linear hopping, in other words to subject them to a wavelengthshift, which does not apply much or at all to the noise. In the FIG. 3example, simple widening of the signal is used to subject the signals tonon-linear hopping. It can be obtained by disposing at the output of theamplifier a section of fiber having zero chromatic dispersion for awavelength close to that of the signals transmitted, for example. Theeffect of such a fiber section is to explode the spectrum intosub-components, as a function of the power and the wavelength. Thechoice of the amplification gain can be optimized to encourage thewidening of the signals transmitted at λ_(c) to maximize the proportionof the signals transmitted around the wavelength λ₁.

[0037]FIG. 3 shows the shape of the widened signal. Note that thissolution is preferably applied to RZ pulses. It has the advantage thatit also peak limits the pulses, in other words reduces the excess powerof the pulses.

[0038] The invention then proposes to filter the signals and the noiseoutside the range of wavelengths centered on λ₁. Such filtering recoversthe original part of the signals transmitted whilst eliminating noise atthe output. In fact, in the first filtration step of FIG. 2, the noiseis eliminated around λ₁. In the second filtration step, only signalsaround λ₁ are recovered, i.e. signals in a range of wavelengths in whichthe noise is low. In fact, the non-linear character of the wideningensures that the noise does not pass much or at all into the rangearound the wavelength λ₁. FIG. 4 shows the shape of the signals obtainedafter filtration around λ₁. The level of the signals around λ₁ isdenoted E′ in the figure. The value of E′ is of the same order as thevalue E of the power of the signal; the loss by shifting and filteringthe signals is compensated in this embodiment by amplifying the signalsused for widening the signals.

[0039] In this embodiment, with values of the order of 1 nm for thedifference between the wavelengths λ₁ and λ_(c), Bragg filters operatingin reflection can be used to filter the signals, or filters known in theart having analogous performance.

[0040]FIG. 5 is a diagrammatic representation of a noise-reducing devicein accordance with the invention. The device has first filter means 1that filter noise around the wavelength λ₁ with no or little powerreduction in the range of wavelengths of the signals transmitted. Thedevice then has wavelength shifting means 2; in the embodiment shown inthe figure, the wavelength shifting means include an amplifier 3followed by a loop of fiber having zero chromatic dispersion at thewavelength λ_(c) of the signals transmitted. As explained above, theshifting means widen the spectrum of the signals transmitted so that thesignals extend into the range of wavelengths in which noise haspreviously been filtered. After these shifting means the device includessecond filter means 5 that filter signals outside the range ofwavelengths around λ₁. Signals with a slightly different wavelength (ahigher wavelength in the example) are recovered at the output of thedevice, with a higher signal/noise ratio.

[0041] To achieve greater shifts, up to a few nanometers, other signalwidening means can be used. In the second embodiment of the invention,described next, optical phase conjugation is used to shift the signalstransmitted. FIG. 6 shows the shape of the signals. It shows in fullline the spectrum of the signal after filtering noise around thewavelength λ₁. The spectrum of a pump is shown in dashed line. The thickline shows the spectrum of the signal obtained around the wavelength λ₁by four-wave mixing of the transmitted signals and the injected pump.

[0042] The second embodiment has the advantage of achieving greaterdifferences between the wavelength λ_(c) and λ₁, in other words agreater wavelength shift. This guarantees that the system has greateracceptance, in particular in the face of variations in the wavelength ofthe source sending the signal. Also, in the case of wavelength divisionmultiplex transmission, the solution of the second embodiment shifts allthe channels as a block, for example by means of a pump with awavelength greater than the greatest wavelength of the channels of themultiplex. The second embodiment applies not only to RZ pulses but alsoto NRZ pulses. Finally, compared to the first embodiment, the secondembodiment does not induce any frequency conversion of the signal, whosespectrum is modified less; this facilitates repetition of filtering andwavelength shifting.

[0043] A third embodiment of the invention proposes to use wavelengthconverters to apply the wavelength shift to the transmitted signals. Inaccordance with the invention, any wavelength converter device known inthe art can be used, not only a converter based on optical conjugationof the signals in four-wave mixing, as in the second embodiment.

[0044] The three embodiments of the invention produce transmittedsignals with a lower noise level around a wavelength shifted relative tothe initial wavelength of the signals. The improvement in thesignal/noise ratio con be of the order of 10 dB.

[0045]FIG. 7 shows the shape of the signals received at the receiver ofa conventional fiber optic transmission system. Time in picoseconds isplotted on the abscissa axis and power in milliwatts is plotted on theordinate axis. The signals are RZ pulses at a bit rate of 10 Gbit/s, andpropagate in 5000 km of optical fiber with an injected power of −4 dBm.The figure shows a high noise level; the figure of merit of the link,measured in a manner known in the art, is of the order of 5.8.

[0046]FIG. 8 shows a similar graph for a link in accordance with theinvention in which noise-reducing means of the type described above areprovided in the receiver. As shown in the figure, the noise level isclearly lower, and the figure of merit is of the order of 18.4.

[0047] In accordance with the invention, a higher power can be injectedinto the link than is possible in the prior art.

[0048] The invention has been described with reference to FIGS. 1 to 8in its simplest application. It is possible to perform transformationsof the type described with reference to FIGS. 1 to 4 several times on alink. Thus, after recovering the signal around the wavelength λ₁ at theoutput of FIG. 4, it can be returned to the wavelength λ_(c). For this,the process is the exact reverse of that described with reference toFIGS. 1 to 4: the first step is to filter noise around the wavelengthλ_(c), if this has not been done during the second filtration step ofFIG. 4; the wavelength of the signals is then shifted from λ₁ to λ_(c)by a process that is the reverse of that of FIG. 3. The signals aroundthe wavelength k are then filtered. This “reversal”, in terms ofwavelength, further reduces the noise level; it has the advantage ofreturning the signals to the initial range of wavelengths.

[0049] The steps described above can also be repeated several times onthe link, increasing or decreasing the wavelength. The number ofrepetitions without changing the direction of variation of thewavelength depends only on the transmission range of the fibers,amplifiers and other components of the transmission system.

[0050] The position of the noise-reducing device or devices inaccordance with the invention depends on the required effects and theembodiment used. Noise can be produced in accordance with the inventionanywhere along the transmission system. The noise is preferably reducedsufficiently early on for the pulses to have a much higher level thanthe noise. A signal/noise ratio of the order of 5 dB/nm or more issuitable.

[0051] In FIGS. 7 and 8, the noise is reduced in accordance with theinvention in the receiver. It could have been reduced sooner; however,this embodiment is particularly advantageous for repeaterless systems.For systems using repeaters a noise-reducing device in accordance withthe invention can be inserted after or before a repeater.

[0052] In the case of a wavelength division multiplex transmissionsystem, noise can be reduced in all the channels of the multiplex at thesame time, as in the second embodiment. If is also possible to operatechannel by channel, after demultiplexing the signal, and then toremultiplex the signal afterwards. This solution has the advantage thatthe respective positions of two channels can be interchanged to preservethe spectral shape of the channels of the multiplex.

[0053] A noise-reducing device in accordance with the invention isadvantageously associated with active devices such as phase or amplitudemodulators for reducing timing jitter. In this way not only the noisebut also the timing jitter of the signal are reduced.

[0054] Of course, the present invention is not limited to the examplesand embodiments described and shown, and lends itself to many variantsthat will be evident to the skilled person. It applies in particular toall fiber optic transmission systems, regardless of the signalstransmitted (RZ or NRZ pulses, solitons or other signals), on a singlechannel or with wavelength division multiplexing. It applies equallywell to repeaterless transmission systems (with no electrically activeelements in the link) and transmission systems using repeaters.

1. A noise-reducing device for a fiber optic transmission system, saiddevice including first means (1) for filtering noise outside the rangeof wavelengths of the signals transmitted, means (2) for shifting thewavelength of the signals transmitted, and second means (5) forfiltering the transmitted signals that have undergone wavelengthshifting.
 2. The device according to claim 1, characterized in that itincludes second wavelength shifting means for returning the signals thathave undergone the second filtration to their initial wavelength.
 3. Thedevice according to claim 1 or claim 2, characterized in that thewavelength shifting means include means (3, 4) for widening the spectrumof the signals.
 4. The device according to claim 1 or claim 2,characterized in that the wavelength shifting means include opticalphase conjugation means.
 5. The device according to any of claims 1 to4, characterized in that the filtration means include a Bragg filter. 6.A fiber optic transmission system including at least one deviceaccording to any one of claims 1 to
 5. 7. A method of reducing noise ina fiber optic transmission system, said method comprising the steps offiltering noise outside the range of wavelength of the signalstransmitted, shifting the wavelength of the signals transmitted, andfiltering the signals transmitted that have undergone wavelengthshifting.
 8. The method according to claim 7, characterized in that itfurther includes a second wavelength shifting step for returning thesignals that have undergone the second filtration step to their initialwavelength.
 9. The method according to claim 7 or claim 8, characterizedin that the wavelength shifting step includes widening the spectrum ofthe signal.
 10. The method according to claim 7 or claim 8,characterized in that the wavelength shifting step includes conjugationof the phase of the signal.